AN-1135
Application Note
One Technology Way P.O. Box 9106 Norwood, MA 02062-9106, U.S.A. Tel: 781.329.4700 Fax: 781.461.3113 www.analog.com
ADC Sampling Information ADM1275/ADM1276/ADM1075
by Paul O’Sullivan
INTRODUCTION
UI/UV
UP/UE
ADC CONVERSIONS
The ADM1275 features current and voltage readback via the
PMBus interface. Figure 1 shows the sampling order. The current
is sampled first, followed by a current register update. Then, the
voltage is sampled followed by a voltage register update. The
register update completes in one clock cycle. On the ADM1275,
the user can select whether to monitor the VIN voltage or the
VOUT voltage but cannot monitor both at the same time.
Figure 1. ADM1275 ADC Updates, Single Sample
V
I
V
I
V
....
I
VIN × I
Figure 3. ADM1276 ADC Updates, Single Sample
As shown in Figure 4, when averaging is enabled, a single power
and energy calculation is completed following the last voltage
and current samples. After these calculations complete, all of
the registers update.
UI/UV
UP/UE
VOUT DISABLED
VIN
I
VIN
I
....
VIN
I
VIN × I
VOUT
VIN × I
VOUT ENABLED
VIN
I
VOUT
....
VIN
I
Figure 4. ADM1276 ADC Updates, Device Averaging
UV
V
VOUT
UP = UPDATE POWER
UE = UPDATE ENERGY
UI/UV
UP/UE
VAUX DISABLED
VIN
I
VIN × I
VAUX ENABLED
VIN
I
VAUX
VIN × I
UP = UPDATE POWER
UE = UPDATE ENERGY
Figure 5. ADM1075 ADC Updates, Single Sample
10384-002
I
I
10384-003
VIN
Figure 2 shows the current and voltage conversions and updates
in the case where averaging is enabled on the device. The current
register updates after the final current sample; the voltage register
updates after the final voltage sample. Again, the voltage conversion corresponds to VIN or VOUT.
UI
VIN × I
10384-005
10384-001
V
UI = UPDATE CURRENT
UV = UPDATE VOLTAGE
I
The ADM1075 is very similar to the ADM1276. The order of
current and voltage conversions is the same. The main difference
between them is that the VOUT measurement on the ADM1276
is a VAUX measurement on the ADM1075, as shown in Figure 5
and Figure 6.
UV
I
VIN
VOUT ENABLED
UI/UV
UP/UE
Figure 2. ADM1275 ADC Updates, Device Averaging
The ADM1276 and ADM1075 both have power metering as
well as voltage and current monitoring. The power is calculated
as VIN × I; therefore, the VIN voltage is always sampled. The
user can opt to sample another ADC voltage input, as well. In
the case of the ADM1276, the VOUT voltage can be measured.
After the power calculation is complete, all of the voltage, current,
power, and energy registers update. See Figure 3 for details.
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VAUX DISABLED
VIN
I
VIN
I
....
VIN
I
VIN × I
VAUX ENABLED
VIN
I
VAUX
....
VIN
I
VAUX VIN × I
Figure 6. ADM1075 ADC Updates, Device Averaging
10384-006
UI
VOUT DISABLED
10384-004
The ADM1275, ADM1276, and ADM1075 all share the same
basic analog-to-digital converter (ADC) core and PMBus interface. There are some subtle differences between the devices with
regard to averaging calculations and ADC register updates. There
are also factors and limitations to consider when reading data
quickly from any of the ADM1275, ADM1276, or ADM1075
devices. This document describes the ADC operation for each
of these devices and describes how to maximize the data rate
from each device, if required.
AN-1135
Application Note
of 115 μs bus free time is needed after the readback command
is completed to allow for the current conversion and register
update to complete. The time required to update the registers is
very small (a single clock period).
Figure 7 shows how the power and energy calculations are
performed.
V1
V2
....
Vn
VAVG
I1
I2
....
In
IAVG
ADM1276
ADM1075
The ADC is locked out from voltage and current updates while
the I2C bus is active (or more accurately, if the device address is
recognized, then lockout occurs between the acknowledge at
the end of the command code and the stop bit). Therefore, the
bus free time includes the time from the end of the last PMBus
command until the command code is recognized in the next
PMBus command.
AVERAGE
POWER
ENERGY
ACCUMULATOR
10384-007
ADM1275
ADM1276
ADM1075
Figure 7. Power and Energy Calculations
The average power or accumulated energy can be read directly
from the device. To perform an energy metering function, an
external microprocessor is required for a real-time clock. The
average power used over a period of time can be calculated
from the delta in energy (ΔE) between the register readbacks
divided by the delta in samples (between the register readbacks).
The total energy can then be calculated by multiplying this
result with the delta in time (Δt) from the time stamps of each
readback.
MICROPROCESSOR
PART REGISTERS
ROLLOVER COUNTER
∆E
÷
kWh
AVERAGE
POWER
SAMPLE COUNTER
10384-008
ENERGY ACCUMULATOR
Figure 8. Calculating Average Power in Time
CONVERSIONS AND TIMING
In the majority of cases where device averaging is enabled, the
ADC timing is not a major concern to the user. However, in the
case where the user wants to read back all of the ADC samples
and postprocess the data externally, care needs to be taken to
ensure all ADC samples are captured.
The first example is the case of the ADM1275 wherein the user
reads back all of the current samples. The total time for a voltage
and current conversion is 305 μs maximum (see Table 1).
Table 1. Maximum Conversion Times (μs)
ADM1275
ADM1276
115
190
Total
(1× averaging)
305
VIN IOUT VOUT PIN Total
(1× averaging)
These values determine the maximum duration of the PMBus
command and, therefore, determine the minimum speed at
which the I2C needs to operate for optimum sampling. For
example, in the case of the ADM1275, when the I2C is optimized
for current readback, set the time between readbacks to 305 μs (see
Figure 9). That means that the portion of the READ_IOUT
command after the command code has been recognized must
be less than 190 μs.
The read current command is three or four bytes of data
following the command code, depending on whether the packet
error checking (PEC) byte is used (see Figure 10). Assuming the
PEC byte is used, and including the acknowledge and stop bits,
there are a total of 40 bits of data after the command code. Consequently, the I2C needs to operate at >210 kHz (40 bits in 190 μs)
to ensure that the remainder of the current readback command is
completed within 190 μs.
In the case where averaging is enabled, set the bus free time to
greater than the voltage conversion time. This setting ensures
that both the current and the voltage registers are updated
before the next readback command (see Figure 2). Set the time
between readbacks to greater than
Number of Averages × 305 μs
ADM1075
IOUT
VIN or VOUT
In the case of a voltage readback, a minimum bus free time of
115 μs guarantees no duplicate samples. The voltage readback
is one sample offset unless there is a bus free time of at least
190 μs to allow the voltage conversion to occur before the next
readback.
146 118 146 16 426
VIN IOUT VAUX PIN Total
(1× averaging)
82 120 82 17 301
Use the CONV function of the GPIO1/ALERT1/CONV pin to
synchronize ADC samples to ensure that the sampling always
occurs at the desired time. This procedure allows for other I2C
activity after the readback command and before the next ADC
sampling sequence initiates. CONV functionality is not
available on the ADM1276.
Therefore, each I2C/PMBus readback command must be spaced
at least 305 μs apart. If the current is being read back, a minimum
Rev. 0 | Page 2 of 4
Application Note
AN-1135
400µs
600µs
ADM1275 ADC STATE
I1
<115µs
<190µs
I2
V2
WAIT I2C
V1
>305µs
I2C READ_IOUT CODE
800µs
1000µs
THE TIME TO SAMPLE CURRENT
AND VOLTAGE ARE FIXED.
I3
V3
<190µs
FREE
DATA
CODE
FREE
DATA
CODE
FREE
DATA
READ I2
READ I1
READ I3
10384-009
200µs
Figure 9. Optimal Current Readback on ADM1275
S
SLAVE ADDRESS
W
DATA BYTE HIGH
S
A
SLAVE ADDRESS
COMMAND CODE
A Sr
SLAVE ADDRESS
R
A
DATA BYTE LOW
A
A
COMMAND CODE
A Sr
SLAVE ADDRESS
R
A
DATA BYTE LOW
A
P
W
DATA BYTE HIGH
A
PEC
A
P
10384-010
A
MASTER TO SLAVE
SLAVE TO MASTER
Figure 10. Read Word and Read Word with PEC
200µs
400µs
600µs
P1
<16µs
ADM1275 ADC STATE
<146µs
<118µs
<146µs
VIN1
I1
VOUT1
800µs
1000µs
THE TIME TO SAMPLE CURRENT,
VOLTAGE AND POWER ARE FIXED.
WAIT
VIN2
I2
1200µs
P3
P2
VOUT2
VIN3
I3
VOUT3
DATA
ADD
DATA
ADD
READ I0
READ I1
DATA
READ I2
10384-011
>426µs
ADD
Figure 11. Register Readback on ADM1276
In the case of the ADM1276 and ADM1075, the update of all
registers occurs at the end of the sampling sequence and after
the power calculation has been completed. Hence, the time
taken by the read command is not important; rather, the critical
time is the time between commands. The time between commands
must be longer than the entire sampling to avoid duplicate
samples (see Figure 11).
Similarly, in the case where averaging is enabled, all register
updates are done at the end of the sampling sequence after the
power calculation. Therefore, set the time between readbacks to
greater than the sampling time. This time can be calculated
from Table 1 using the following formula:
Rev. 0 | Page 3 of 4
Min Readback Time = [Number Averages × (Voltage
Sample + Current Sample + Optional Voltage Sample)] +
Power Calculation Time
AN-1135
Application Note
CONCLUSION
The ADM1275, ADM1276, and ADM1075 provide different
combinations of voltage, current, and power monitoring. There
are subtle differences between the order of sampling, where
ADC register updates occur, and the ADC timing on each
device. Consider these differences to optimize the PMBus
transactions for maximum data rate and to avoid duplicate
samples.
The minimum I2C data rate and minimum bus free time (for
the ADM1275 only) are the critical specifications when optimizing
data readback, for example, where postprocessing of data is con-
ducted externally to the device (that is, in a microprocessor). In a
case such as this, the user needs to set single average sampling
and must readback every sample from the device. In most cases
where device averaging is used, this level of readback
optimization is not required.
When timing is critical, the CONV function of the GPIO1/
ALERT1/CONV pin helps to synchronize the sampling. On
the ADM1275, where some bus free time is required for optimal
readback, use the CONV pin function to delay ADC sampling to
allow for other I2C activity following the initial readback
command.
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AN10384-0-8/12(0)
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